1. Field of the Invention
The present invention relates to a method and apparatus for efficiently controlling congestion of nodes in a wireless mobile ad-hoc network (hereinafter, referred to as an “ad-hoc network”) to which much attention has been given as a networking technology in a ubiquitous environment due to the development of wireless connectivity and mobile technology.
2. Description of the Related Art
At present, there is no standard media access method dedicated to an ad-hoc network. Therefore, the IEEE 802.11 MAC protocol is generally used as a media access method in ad-hoc networks. A transmission control protocol (TCP) is widely used as a transmission protocol in ad-hoc networks.
The TCP is a standard transmission protocol which is the most widely used Internet protocol. The TCP has been proposed for reliable data transmission and efficient congestion control in cable networks. The TCP provides a method of controlling congestion based on a congestion window by taking into consideration a round-trip time (RTT) which is a packet round-trip time between a transmission node (sometimes, referred to as a source node) and a reception node (sometimes, referred to as a destination node).
When initial data transmission is smoothly achieved, the transmission amount is increased gradually while increasing the size of the congestion window. However, if packet loss occurs, congestion is assumed to occur at an intermediate node on a path between the transmission node and the reception node. Therefore, the size of the congestion window is decreased rapidly at a TCP transmitter side (an additive-increase-multiplicative-decrease (AIMD)
In the cable network, which has a reliable transmission network, the method of controlling congestion in the TCP is superior. However, in the ad-hoc network, in which the reason for the packet loss is fundamentally different from that of the cable network, performance is remarkable decreased. In the ad-hoc network, much of the packet loss is caused due to channel contention which is caused by excessive traffic that exceeds channel capacity.
In the ad-hoc network, in which the reason for the pack loss is different from that of the cable network, there is a limit to the amount by which the performance can be improved by the conventional method of controlling congestion in the TCP. Accordingly, a new method of controlling congestion for only the ad-hoc network is necessary.
An example of a conventional method of controlling congestion in the ad-hoc network is a method of restricting the size of the congestion window in the TCP using the number of hops between the transmission node and the reception node. The conventional AIMD method of controlling congestion in the TCP generates temporary excessive traffic in the ad-hoc network so as to cause serious packet loss. Therefore, this method has been proposed to decrease traffic burstness by restricting the maximum size of the congestion window to a reasonable level. In this method, since the size of the congestion window is restricted to one quarter of the number of hops between the transmission node and the reception node, traffic overload can be avoided. Accordingly, the packet loss can be significantly decreased. However, the restriction in the size of the congestion window lowers the throughput of nodes, so that the overall performance in the network is lowered.
As another example of the conventional method of controlling congestion in the ad-hoc network, there is a method of controlling congestion by taking into consideration a path failure which can occur due to nodes having mobility. In other words, when the packet loss occurs due to path failure, the data transmission is interrupted so that path recovery can be performed. After the path recovery is achieved, the transmission node restarts the data transmission to prevent possible packet loss during the path recovery. This method provides a solution to the path failure by taking into consideration the mobility of nodes in the ad-hoc network. However, the fundamental cause of the packet loss has not been taken into consideration.
Another example of a conventional method of controlling congestion in the ad-hoc network is a method of adaptively controlling congestion by classifying the packet loss that occurs in the ad-hoc network according to the cause of the packet loss. In other words, the packet loss is classified into packet loss due to path failure, packet loss due to physical errors caused by wireless media, and packet loss due to excessive traffic. The method of controlling congestion is applied differently depending on the cause. This method not only spends a long time finding the cause of the packet loss but it also is difficult to accurately find the cause. Therefore, this method is in danger of misjudging the cause, and a long time is required to control congestion.
Another example of a conventional method of controlling congestion in the ad-hoc network is a method of controlling congestion using transmission amount-based control (sometimes, referred to as a rate-based control), which is different to the congestion window-based control provided by the TCP. Unlike the conventional method of controlling congestion based on the congestion window, this method can avoid traffic overload to loosen the channel contention. However, since control of the transmission amount is performed on the basis of the RTT between the transmission node and the reception node, the overall throughput in the network may decrease.
As described above, the methods of controlling congestion proposed for the ad-hoc network do not take into consideration the channel contention which is the most immediate and important cause of packet loss.